Conventional topologies for class D amplification (sometimes called class T amplification) are based on a system where there are two DC supply rails carrying inverse DC voltages, one at voltage +V and the other at −V. A pair of switches are provided, each of which closes between a common junction and one of the supply rails. When one switch is open the other is closed such that the voltage at the common junction is either +V or −V.
It should also be noted that since the system must be operated such that both switches are never closed at the same time, thereby causing a direct short circuit, it is necessary that for a short period of time, both switches are open. For the purposes of this description, however, the voltage at the common junction is either +V or −V.
A control circuit opens and closes the switches, causing the voltage at the common junction to change back and forth between +V and −V. The input of a low pass filter is connected to the common junction to remove the high frequency switching components from the signal and the output of the filter is connected to the transducer to reproduce the amplified output signal. The transducer can also provide the effect of a low pass filter such that a separate filter is not required. This is used for audio amplifiers, power inverters, motor control and many other applications.
There are many different techniques for determining the control circuit that operates the switches. These range from a fixed frequency pulse width modulation (PWM) to variable frequency direct digital modulation.
The conventional class D topologies have several drawbacks. In order to control the switches, the control circuit must be able to withstand the voltage on the rails, which can be quite high, and yet switch very fast. The switches are usually field effect transistors (FETs) or bipolar junction transistors (BJTs) which require that the control be within a few volts of the source or emitter.
The conventional class D topology does not provide isolation between the incoming power supply and the output connections. This means that for any application running off the AC power lines, a power supply is required that provides this isolation.
The conventional class D topology is only able to get the output voltage to swing between the +V and −V available at the supply rails. If the available supply voltage is too low or too high, then an additional DC/DC converter is required to generate the appropriate voltages.
Unless the conventional class D topology is run in a bridged configuration, a problem known as supply pumping may be seen. When one of the switches in the pair closes, the current in the filter inductor increases. When the first switch opens, the other has to close or a voltage spike results. Since the inductor current cannot instantaneously change, the inductor will now draw the current from the other supply. This converts power from one supply to the other so as one supply is loaded, the other will need to absorb power.